There was an bug in the Composite method so I had to publish a new release. Please upgrade to Magick.NET 7.0.0.0002. I have added a new 'Tile' method to MagickImage to make life easier for you. Your command now translates to this:
using (MagickImage image = new MagickImage(sourcePath)) { image.Resize(259, 384); image.Sharpen(0.75, 1); using (MagickImage watermark = new MagickImage("C_watermark.png")) { image.Tile(watermark, CompositeOperator.Dissolve, "11"); image.Write("Preview-CSharp.jpg"); } }